1. Field of the Invention
The invention relates to a control apparatus and a control method for a hybrid vehicle including plural driving sources.
2. Description of the Related Art
As a hybrid vehicle, for example, a vehicle which includes a motor or motor/generator as a power source in addition to an internal combustion engine is known. In this example of the hybrid vehicle, an engine rotational speed is controlled by the motor or the motor/generator connected to a planetary gear mechanism so that the internal combustion engine is operated at an optimal operating point, using a differential effect of the planetary gear mechanism. Also, at the time of acceleration, excess or deficiency of driving force or driving force required by an engine brake is compensated for by the motor or the motor/generator. Further, at the time of deceleration, energy is regenerated, whereby exhaust gas discharged from the internal combustion engine is reduced, and fuel efficiency is improved at the same time.
As an example of the hybrid vehicle, a drive apparatus for a hybrid vehicle is disclosed in Japanese Patent Application Publication No. JP-A-2002-225578. In the drive apparatus, an engine, a first motor/generator, and an output member are connected to each other through a power splitting mechanism, and a second motor/generator is connected to the output member. Output of the engine is split and transmitted to the output member and the first motor/generator by the power splitting mechanism. Also, the second motor/generator is driven by electric power generated by the first motor/generator. Thus, torque of the output member is increased or decreased. Further, the engine rotational speed is maintained on an optimal fuel efficiency line by controlling the rotational speed of the first motor/generator. Thus, the fuel efficiency can be improved.
Meanwhile, it is conceivable to manually change the rotational speed of the first motor/generator. By manually changing the rotational speed of the first motor/generator, the engine rotational speed can be made equal to a rotational speed required by a driver, and driveability can be improved. However, when downshifting is performed according to manual operation without changing an operational state of an accelerator pedal, the engine rotational speed is increased while the engine output is maintained at a constant value. Therefore, the output toque of the engine is relatively decreased. As a result, torque that is directly transmitted from the engine to an output shaft is reduced. Therefore, in order to avoid or suppress a decrease in the driving force, the required torque of the second motor/generator based on the required output of the second motor/generator becomes large. Accordingly, size of the second motor/generator needs to be increased for providing the required torque of the second motor/generator. Otherwise, the second motor/generator may not be able to provide the required output due to a limit of the rotational speed of the second motor/generator.